Add an order-only dependency on the build number file

Remote execution and other tools can be confused by references to
build_number.txt without a dependency.  Add an order-only dependency,
which maintains the current behavior.

Test: BUILD_NUMBER=1 && m aapt && aapt version # shows 1
      BUILD_NUMBER=2 && m aapt && aapt version # shows 1
      rm out/soong/.intermediates/frameworks/base/tools/aapt/aapt/linux_glibc_x86_64/aapt
      BUILD_NUMBER=2 && m aapt && aapt version # shows 2
Change-Id: Icfa98d6840b1dc2e273ba29c33011635d1cf93b1